The process steps for the production of polylactide concern for example direct polycondensation of lactic acid, thermal depolymerisation of polylactide into dilactide, cleaning of the dilactide by means of distillation, rectification or crystallisation, polymerisation and demonomerisation. The vapours from these process steps occur under reduced pressures or vacuum which can be between 5 mbar and 200 mbar. According to the process step, they contain water, lactic acid, dilactide and lactoyllactate in different compositions. These components can be condensed as far as possible for protection of the vacuum pumps but also for economic process reasons and be returned into the process.
The condensation of dilactide-containing vapours on cooled surfaces of condensers presents difficulties. An aerosol is produced which cannot be precipitated with normal means, such as drop or mist depositors, but leaves the condenser with the non-condensable residual gas and thus passes into the vacuum pumps which withdraw and condense this residual gas.
This problem is compounded inasmuch as the vapours from the process steps for the production of polylactide contain inert gases, such as air or nitrogen. In vacuum pumps, the dilactide aerosol leads within a short time, as a result of increased wear and tear of metallic surfaces, such as rotary pistons, rotary valves, rotary plungers and the housings thereof, to mechanical destruction. A further problem is the conversion of the dilactide by the water vapour which is always still contained in the residual gas into lactoyllactate which, together with the likewise still entrained lactic acid residues, attack these metallic surfaces by corrosion and permanently destroy them.
Indirect condensation on cooled surfaces is generally preferred since, in contrast to direct condensation with cold liquids, it introduces no additional substances, possibly extraneous, into the process and does not increase the quantity of condensate.
U.S. Pat. No. 5,266,706 describes a process for recovering a cyclic ester, such as lactide, from a gas flow which contains the lactide and hydroxyl group-containing impurities, such as water and hydroxycarboxylic acids, by washing the gas flow with a solvent which is not miscible with water, such as non-polar hydrocarbons, cylcoaliphatic hydrocarbons or halogenated hydrocarbons. The temperature is thereby adjusted during the washing such that the cyclic ester and the hydroxycarboxylic acid is removed from the gas flow, whilst water remains in the gas flow and is discharged with the latter. The crude mixture of cyclic ester and acid is separated from the solvent and cleaned in that the acid is extracted therefrom. However, it is disadvantageous with the mentioned process that the lactide which is cleaned in this manner contains, after the processing, process-foreign solvents, i.e. solvents which do not correspond to the educts contained in the original lactide flow and which must be removed again subsequent to the method by means of complex steps. This involves high complexity and costs.